The solution became colorless? Give it a shake, and it will turn blue again!
- Put on protective gloves and eyewear.
- Conduct the experiment on the plastic tray.
- Do not allow chemicals to come into contact with the eyes or mouth.
- Keep young children, animals and those not wearing eye protection away from the experimental area.
- Store this experimental set out of reach of children under 12 years of age.
- Clean all equipment after use.
- Make sure that all containers are fully closed and properly stored after use.
- Ensure that all empty containers are disposed of properly.
- Do not use any equipment which has not been supplied with the set or recommended in the instructions for use.
- Do not replace foodstuffs in original container. Dispose of immediately.
- In case of eye contact: Wash out eye with plenty of water, holding eye open if necessary. Seek immediate medical advice.
- If swallowed: Wash out mouth with water, drink some fresh water. Do not induce vomiting. Seek immediate medical advice.
- In case of inhalation: Remove person to fresh air.
- In case of skin contact and burns: Wash affected area with plenty of water for at least 10 minutes.
- In case of doubt, seek medical advice without delay. Take the chemical and its container with you.
- In case of injury always seek medical advice.
- The incorrect use of chemicals can cause injury and damage to health. Only carry out those experiments which are listed in the instructions.
- This experimental set is for use only by children over 12 years.
- Because children’s abilities vary so much, even within age groups, supervising adults should exercise discretion as to which experiments are suitable and safe for them. The instructions should enable supervisors to assess any experiment to establish its suitability for a particular child.
- The supervising adult should discuss the warnings and safety information with the child or children before commencing the experiments. Particular attention should be paid to the safe handling of acids, alkalis and flammable liquids.
- The area surrounding the experiment should be kept clear of any obstructions and away from the storage of food. It should be well lit and ventilated and close to a water supply. A solid table with a heat resistant top should be provided
- Substances in non-reclosable packaging should be used up (completely) during the course of one experiment, i.e. after opening the package.
FAQ and troubleshooting
The water may be anywhere from lukewarm to hot. Its temperature only influences the reaction rate. The warmer the water you use, the faster the first discoloration of the solution.
The liquid will “work” as long as there is oxygen in the flask and until the glucose supply lasts. So you can still play with it even the next day!
Make a solution of the reductant (glucose).
Add the oxidizer (methylene blue).
The reaction only works in basic medium. Add NaOH to create it.
Methylene blue is oxidizing glucose. It is transforming into its reduced form—leucomethylene blue, which is colorless.
Leucomethylene blue is easily oxidized by the oxygen in the air. Shake the flask to saturate the solution with oxygen.
Blue solution in the flask becomes colorless. Shaking the flask turns the solution blue again!
Dispose of solid waste together with household garbage. Pour solutions down the sink. Wash with an excess of water.
Why does the solution become colorless?
Initial glucose contains participants for a potential chemical reaction. On the one hand, it is glucose itself, which is glad to provide electrons. On the other hand, it is oxygen dissolved in water that is willing to accept these electrons. Interestingly enough, though, oxygen isn't that ready to interact with glucose. And methylene blue can help the situation here: this colored compound acts as a carrier in our experiment. It drags electrons from glucose and passes them to oxygen. However, at a certain point there turns to be no more oxygen in the solution. And then, methylene blue that has taken electrons from glucose has nowhere to give them to. And at such state, methylene blue becomes colorless, as we can see.
Why does the solution turn blue again?
We can saturate the solution again with oxygen from the air above the solution. After the flask has been shaken, oxygen from the air dissolves in the solution. Then, the reaction can proceed again, until all the oxygen available in the solution is spent. However, this trick cannot be repeated endlessly. Since the flask is tightly sealed, sooner or later all the oxygen from the air will be depleted, and the solution will then remain colorless. Nevertheless, the process can be reactivated by opening the flask to let extra air in.
Why did we add alkali to the glucose aqueous solution?
By adding sodium hydroxide NaOH aqueous solution, we created an alkaline environment. This special requirement must be met for methylene blue to accept electrons from glucose. Otherwise, the reaction would not proceed, and the solution would remain blue. You may check it by running the experiment without NaOH.
Why is it so important to seal the flask tightly?
First of all, it is for convenience. This way you may shake the flask without fearing its contents would spill out.
Moreover, by sealing the flask we prevent the ambient air from getting into the flask. Therefore, oxygen from the ambient air will not have access to our solution as well. Therefore, color recovery is only possible after shaking the flask (see Why does the solution turn blue again?). The most diligent experimentalists have probably noticed that after the first shake blue coloring doesn't disappear completely. It remains at the border between the solution and air in the flask (along so-called meniscus) and forms a nice blue fringe. The same would happen if the flask is left opened. This is caused by oxygen present in high concentration in the air above the solution. Oxygen penetrates through the liquid-gas interface and transforms methylene blue into its colored form. However, when oxygen is being gradually depleted in the flask, this border gets thinner and finally disappears.
Milk contains lactose, so try repeating the experiment using fresh milk instead of glucose solution! Share your experience online in your scientific blog.
Another compound you can easily obtain for this chemical reaction is vanillin. It is often used as a flavoring agent in food industry and can be found in a grocery store. Vanillin also contains an aldehyde –CHO group, and the reaction would proceed in the way described above.